This invention relates to a flowmeter of the xe2x80x9chot wirexe2x80x9d variety, and more particularly, to a circuit for temperature compensating such a flowmeter.
Mass air flow meters used in automotive vehicles often are of the constant temperature anemometer type. In these meters, a sensing element is electrically heated to a constant temperature differential above the ambient air temperature. Heat is convectively removed from the element by the airflow and the current flowing in the element replaces the heat lost by convection. As the mass air flow varies, the current required to maintain the requisite temperature also varies such that the current is a known function of the mass air flow.
Typically, a xe2x80x9chot wirexe2x80x9d type flowmeter includes a self-heated sensor resistor having a resistance RH which is a function of its temperature. In turn, the temperature of the heated resistor is determined at least in part by the difference between the heat generated within the heated resistor as a function of the voltage applied across the resistor and the heat transferred away from the heated resistor as a function of the amount of cooling fluid flow past the resistor. In addition, it is usual for a xe2x80x9chot wirexe2x80x9d flowmeter to include an ambient temperature sensing resistor having a resistance RA determined by the ambient temperature of the flowing fluid.
Hot element anemometers frequently use a Wheatstone bridge configuration for the sensing elements. As shown in FIG. 1, it is commonplace to employ a flow sensing resistor RH in one leg of a bridge 10 and an ambient temperature sensing resistor RA in another leg of bridge 10. In a bridge-type xe2x80x9chot wirexe2x80x9d flowmeter, the self-heated resistor RH and the ambient temperature resistor RA are connected within a bridge circuit across which a voltage Vb is developed. In terms of fundamental structure, the bridge circuit includes a signal side for deriving a signal voltage VRL which is a voltage divided function of the bridge voltage Vb as determined at least in part by the resistance RH of the sensor resistor in ratio to the resistance RL of a power dissipating resistor. The bridge circuit further includes a reference side for defining a reference voltage Vr which is a voltage divided function of the bridge voltage Vb as determined at least in part by the sum (RA+R1) of the resistance RA of the ambient resistor plus the resistance R1 of a ballast resistor in ratio to the resistance R2 of a calibration resistor.
It is common in a bridge-type flowmeter to drive the bridge circuit with an operational amplifier which compares the signal voltage VRL with the reference voltage Vr. More specifically, the amplifier is responsive to the difference between the two voltages VRL and Vr to alter the bridge voltage Vb thereby correspondingly altering the voltage applied across the self-heated resistor so as to change the heat generated within the resistor. As a result, the temperature of the heated resistor and its related resistance RH are modified such that the signal voltage VRL is equalized with the reference voltage Vr. Under these circumstances, the bridge voltage Vb is indicative of the amount of fluid flow.
Resistor R2 in the lower arm of the bridge completing the bridge configuration with resistor R1 in series with the ambient sensing resistor RA is useful in bridge balance and calibration. The bridge values are selected so that the bridge will be balanced when the flow sensing resistor RH is at a prescribed temperature differential above the ambient temperature. As airflow changes tend to result in resistor RH changes, the bridge tends to unbalance and the amplifier makes a correction in the applied bridge voltage to restore the resistor temperature differential and thus the bridge balance. The applied bridge voltage Vb therefore varies with airflow and is useful as a measure of mass airflow.
The hot-wire type of sensor has several limitations. In particular, the RH and RA resistances are not consistent enough to have single value resistors to form the Wheatstone bridge described above. To compensate for the RH and RA resistance value variations, thick film resistors are laser trimmed to individually match corresponding RH and RA values. Furthermore, the Wheatstone bridge requires a costly ambient temperature sensor having a resistance vs. temperature characteristic similar to the heated sensor. Thus, it would be desirable to temperature compensate a hot wire anemometer without costly laser trimmed resistors and an ambient temperature sensor.
Disclosed herein is a method and apparatus for measuring the amount of flow of a flowing medium. The apparatus includes a bridge circuit across which a bridge voltage Vb is developed such that the magnitude of the bridge voltage Vb is indicative of the amount of flow. The bridge circuit has a signal side for deriving a signal voltage VRL which is a voltage divided function of the bridge voltage Vb as determined at least in part by the resistance RH of a self-heated resistor in ratio to the resistance RL of a power resistor where the resistance RH is related to the temperature of the heated resistor as determined at least in part by the difference between the heat generated within the heated resistor as a function of the voltage applied across the heated resistor and the heat transferred away from the heated resistor as a function of the amount of fluid flow. The bridge circuit also has a reference side for defining a reference voltage Vr which is a voltage divided function of the bridge voltage Vb as determined at least in part by a resistance Rp of a potentiometer. The bridge circuit further includes an amplifier responsive to the difference between the signal voltage VRL and the reference voltage Vr for altering the bridge voltage Vb to maintain the heat generated within the self-heated resistor thereby maintaining its temperature and related resistance RH so as to equalize the signal voltage VRL and the reference voltage Vr.
The method for temperature compensation of a constant temperature anemometer described above further includes generating a temperature reference voltage indicative of ambient temperature from a thermistor circuit; receiving the uncompensated bridge voltage and the temperature reference voltage in a conditioning circuit configured to process the uncompensated bridge voltage and the temperature reference voltage; and generating a compensated bridge voltage with respect to ambient temperature. The compensated bridge voltage indicative of fluid flow across resistor RH in an ambient temperature range.
The above described and other features are exemplified by the following figures and detailed description.